Display centering system

ABSTRACT

A display centering system comprises a cathode ray tube having a metalized screen which is etched to provide at least a pair of electrically discrete sectors. The split screen acts as a target anode; and is operated at a potential to collect all primary and secondary electrons. The beam current is momentarily increased at predetermined points of the raster independently of the normal video signals; and the differential anode currents between the two halves of the split screen are measured at such points of the raster to determine positioning errors. In order to avoid excitation of the phosphors to a visible level, the points in the raster at which the beam current is momentarily increased is successively varied from field to field and from frame to frame. The metalized layer of the screen is preferably split along a line passing through the center of the display so that the display may be centered with the highest accuracy. Information can be readily displayed at the center of the screen, since at any given point the video signal will only occasionally be affected by the pulse of increased beam current which is used to establish the center of the display.

United States Patent [191 Mann [4 1 Apr. 3, 1973 [54] DISPLAY CENTERINGSYSTEM [57] ABSTRACT Inventofi George Mann, Galeshead A displaycentering system comprises a cathode ray 97YA, England tube having ametalized screen which is etched to pro- [73] Assigneez United AircraftCorporation, East vide at least a pair of electrically discrete sectors.The Hartford Conn. split screen acts as a target anode; and is operatedat a potential to collect all primary and secondary elecl Filedl p 19,1971 trons. The beam current is momentarily increased at [21] AppL No:134,961 predetermined points of the raster independently of the normalvideo signals; and the differential anode currents between the twohalves of the split screen are U-S- CL R, R measured at uch points ofthe aster to determine [51] Int. Cl. ..H0lj 29/70 positioning errors InOrder to avoid excitation f the Field of Search "315/21 22, 23, 27 27 TDphosphors to a visible level, the points in the raster at which the beamcurrent is momentarily increased is [56] References cued successivelyvaried from field to field and from frame UNITED STATES PATENTS toframe. The metalized layer of the screen is preferably split along aline passing through the center 3,609,219 1/1970 Diehl ..315/27 TD ofthe display so that the display may be centered with 2,855,540 HOOVCI'et a]. ..315/21 R the highest a curaxa Information can be readily dis-2,630,548 3/1953 Muller ..315/21 R Primary ExaminerCarl D. QuarforthAssistant ExaminerJ. M. Potenza Attorney-Melvin Pearson Williams playedat the center of the screen, since at any given point the video signalwill only occasionally be affected by the pulse of increased beamcurrent which is used to establish the center of the display.

27 Claims, 5 Drawing Figures VERY- sweep PATENTEUAPR3 I975 BKV.

sum 2 OF IN VE A 102 76 F Geo/ye Map HT TORNEYS DISPLAY CENTERING SYSTEMBACKGROUND OF THE INVENTION center of the display adjacent the center ofthe tube.

Furthermore, the beam current is increased at the same points in theraster so that it is necessary to mask the phosphors in these areasadjacent the edges of the display, thus obscuring the field of view.

SUMMARY OF THE INVENTION One object of my invention isto provide adisplay centering system employing a split metalized screen as thetarget anode.

Another object of my invention is to provide a display centering systemin which the metalized screen is split along a line passing through thecenter of the display.

A further object of may invention is to provide a display centeringsystem in which the cathode ray tube beam current is momentarilyincreased at points in the raster which vary from field to field andfrom frame to frame and which lie along the line of splitting of themetalized screen.

Other and further objects of my invention will appear from the followingdescription.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings whichform part of the instant specification and which are to be read inconjunction therewith and in which like reference numerals are used'toindicate like parts in the various views: I

FIG. 1 is a schematic view showing a first embodiment of my invention;

FIG. 2 is a fragmentary schematic view showing a second embodiment of myinvention;

FIG. 3 is a fragmentaryschematic view showing a third embodiment of myinvention;

FIG. 4 is a fragmentary schematic view showing a fourth embodiment of myinvention;

FIG. 4a is a diagrammatic view showing the pattern of beam currentmodulation provided in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 of thedrawings, a cathode ray tube 10 is provided with a metalized-screenwhich may comprise a thin and transparent aluminized layer between thetransparent glass face of the tube and the phosphor layer. The metalizedlayer is etched along a horizontal line to provide upper quadrants 12and and lower quadrants 14 and 18. The metalized layer is also etchedalong a substantially vertical line to provide left quadrants 12 and 14and right quadrants 15 and 18.

In FIG. 1, quadrants l2 and 15 determine the lateral position of anupper portion of the display; quadrants l4 and 18 determine the lateralposition of a lower portion of the display; and quadrants l5 and 18determine the vertical position of a central portion of the display.Quadrants l2 and 14 are not used to determine'the vertical position ofthe display, since they abut along a line which is remote the centerthereof.

The embodiment of FIG. 1 is adapted for horizontal and verticalcentering of the display as well as correcting for rotational errors.

A 10 kilovolt source of anode potential 26 is connected to center tapsof the primary windings of transformers 20, 22, and 24. The terminals ofthe primary winding of transformer 20 are connected to respectivequadrants 12 and 15. The terminals of the primary winding of transformer22 are connected to respective quadrants 15 and 18. The terminals of theprimary winding of transformer 24 are connected to respective quadrants14 and 18. Anode source 26 is connected to the positive terminal of a 25volt battery 26a, the negative terminal of which is connected to theinternal aquadag coating on the side walls of the cathode ray tube. Oneterminal of the secondary winding of each of transformers 20, 22, and 24is grounded. The other terminal of the secondary winding of transformer20 is coupled through a gate 30 to a low-pass R-C filter 36 having atime-constant of 8 microseconds. The other terminal of the secondarywinding of transformer 22 is coupled through a gate 34 to a low-pass R-Cfilter 40 having a time-constant of l as. The other terminal of thesecondary winding of transformer 24 is coupled through a gate 31 to alow-pass R-C filter 38 having a time-constant of 8 us. The outputs offilters 36 and 38 are applied to a differential amplifier 46 and to asumming amplifier 42. The output of differential amplifier 46 is coupledto a 0 input of cathode ray tube 10 which controls rotation of thedisplay. The display may be rotated by mechanically rotating thedeflection yoke. Preferably, the rotation input 0 comprises a windingdownstream of the deflection yoke which provides an axial fieldcomponent as shown in the co-pending application of David E. Wadlow forDynamic Rotation of Cathode Ray Tube Display, Ser. No. 105,918, filedJan. 12, 1971. The output of summing amplifier 42 is coupled through aresistor 44 to the horizontal deflection input H of cathode ray tube 10.The output of filter 40 is applied to an amplifier 48, the output ofwhich is coupled through a resistor 50 to the vertical deflection inputV of cathode ray tube 10. A source of video signals 93 is coupledthrough a resistor 94 to the intensity input I of tube 10.

A 30,720 Hz oscillator 52 is coupled to a divide-bytwo flip-flop 53. Aplus output of flip-flop 53 is applied to a differentiating circuit 54.The output pulses of differentiating circuit 54 are coupled forwardlythrough a diode 55 to the indexing input of a recycling eight-bit binarycounter 58 and to the retrace input of a horizontal sweep generator 56.Sweep generator 56 provides a triangular wave form which rises from zeroto volts and then retraces to zero again. The retrace period for sweepgenerator 56 is 1 us. Sweep generator 56 provides a blanking output (notshown) which is applied to inhibit the intensity input of tube 10 andthus prevent retrace lines from appearing in the display. The repetitionfrequency of the triangular wave form provided by sweep generator 56 is15,360 Hz. Counter 58 provides outputs proceeding 0, 1, 2, 126, 127,128, 129, 254, 255, and 0 again. The output of counter 58 is coupled toa circuit 60 which detects a count of 0 and to a circuit 59 whichdetects a count of 128. Counter 58 provides bits having respectiveweights of l, 2, 4, 8, 16, 32, 64, and 128. The output of circuit 60 isapplied to a differentiating circuit 61, the output of which is coupledforwardly through a diode 62 to a divide-by-two flipflop 92 and to theretrace input of a 60 Hz vertical sweep generator 63. Sweep generator 63provides a sawtooth wave form having a linearly rising ramp and a rapidretrace.

The outputs of sweep generator 63 and divide-bytwo flip-flop 92 arecoupled through respective summing resistors 70 and 69 to one terminalof a capacitor 71, the other terminal of which is coupled to thevertical deflection input V of tube 10. The output of divide-by-twoflip-flop 92 is coupled forwardly through a diode 65 to index arecycling six-bit binary counter 66. Counter 66 provides countsproceeding 1, 62, 63, and 0 again. Counter 66 provides output bitshaving respective weights of l, 2, 4, 8, l6, and 32. The six output bitsof counter 66 are coupled to one input of a digital comparator 78. Thesix least significant bits of counter 58 are coupled to the other inputof comparator 78.

The output of sweep generator 56 is coupled through a capacitor 57 tothe horizontal deflection input H of tube and is further connected to aplus input of a differential amplifier 72. The output bits of counter 66are applied to a digital-to-analog converter 67 which is supplied with avolt input. As the output of counter 66 varies from 0 to 63", the outputof converter 67 correspondingly varies from zero volts to +20 volts. Theoutput of converter 67 is applied to one input of a summing amplifier68, the other input of'which is supplied with a constant +30 volts. Theoutput of summing amplifier 68 is applied to a minus" input ofdifferential amplifier 72. The output of circuit 59 is coupled to oneinput of AND circuit 73. The output of divide-by-two flip-flop 92 iscoupled to the other input of AND circuit 73. The outputs of circuits 72and 73 are applied to respective inputs of AND circuit 74, the output ofwhich is coupled to a differentiating circuit 75. The output ofdifferentiating circuit 75 is coupled forwardly through a diode 76 totrigger a monostable multivibrator 77 which provides an output pulse of0.25 us duration. The output of multivibrator 77 enables gate 34 and isapplied to one input of OR circuit 91. The output of OR circuit 91 iscoupled to the intensity input 1 of tube 10.

The 32 bit output of counter 58 is coupled to an enabling input of ANDcircuit 80 and to an inhibiting input of AND circuit 79. The 64 bitoutput of counter 58 is applied to an inhibiting input of AND circuit 80and to an enabling input of AND circuit 79. The outputs of AND circuits79 and 80 are coupled through an OR circuit 81 to one input of ANDcircuit 82. The output of comparator 78 is applied to a second input ofAND circuit 82; and the minus output of divide-by-two flip-flop 53 iscoupled to a third input of AND circuit 82. The output of AND circuit 82is coupled to inputs of AND circuits 83 and 84. The 128 bit output ofcounter 58 is applied to the other input of AND circuit 84 and to aninhibiting input of AND circuit 83. The outputs of AND circuits 83 and84 are coupled to respective differentiating circuits 85 and 86. Theoutput of differentiating circuit 85 is coupled forwardly through adiode 87 to trigger a monostable mu1tivibrator 89 which provides anoutput pulse of 1 us duration. The output of differentiating circuit 86is coupled forwardly through a diode 88 to trigger a monostablemultivibrator 90 which provides an output pulse of 1 us duration. Theoutputs of multivibrators 89 and 90 are coupled to further inputs of ORcircuit 91. Multivibrator 89 enables gate 30; and multivibrator 90enables gate 31.

In operation of the circuit of FIG. 1, the display provides 60 fieldsper second, each of 256 lines, and 30 frames per second, each of 512lines. Interlaced scanning is provided by flip-flop 92 which couplessmall positive and negative currents through resistor 69. This variesthe vertical position of the horizontal scan lines for alternate fields.

Battery 26a preferably provides a potential of at least 10 to 15 voltsso that metallized quadrants 12, 14, 15, and 18 of the screen willcollect substantially all of the beam current. Secondary electronsemitted from the screen by virtue of the high voltage through which thebeam of electrons is accelerated will be attracted back to the screenand will not be collected by the aquadag coating on the inner side wallsof the tube.

Once each frame multivibrator 77 provides a bright-up pulse which iscoupled through OR circuit 91 to the intensity input of tube 10. Thisbright-up pulse is provided when the output of horizontal line counter58 is 128 and when the output of divide-bytwo flip-flop 92 is positive.Thus during the 128th line of the first field of each frame, AND circuit73 provides an output. This horizontal sweep line passes substantiallythrough the center of the display. The position along this horizontalsweep line at which multivibrator 77 provides a bright-up pulse iscontrolled by differential amplifier 72. During the course of 64 frames,the bright-up pulse provided by multivibrator 77 varies from position .1to position K. Position .1 is located 37.5 percent of display width fromthe left-hand margin; and position K is located 62.5 percent of displaywidth from the left-hand margin. For one frame the output of framecounter 66 will be 0; and the output of converter 67 will be zero volts.The output of summing amplifier 68 is thus 30 volts. Since sweepgenerator 56 provides a maximum output of 80 volts, the output ofdifferential amplifier 72 will become positive at a point during thehorizontal sweep which is 30/80 0.375 of display width from theleft-hand margin. When the output of differential amplifier 72 becomespositive, AND circuit 74 provides an output which is coupled throughdifferentiating circuit and diode 76 to trigger multivibrator 77 andthus provide bright-up pulse .1. For the subsequent frame, the output ofcounter 66 will be 1"; and the output of converter 67 will be 20/640.3125 volt. Summing amplifier 68 will provide an output of 30.3125volts. The output of differential amplifier 72 becomes positive at aslightly later point during the horizontal sweep. With a l output fromcounter 66, the bright-up pulse provided by multivibrator 77 will beshifted to the right by 0.3125/ 0.39 percent of display width, whichcorresponds to a position of 37.89 percent of display width from theleft-hand margin. Since the horizontal sweep frequency is 15,360 Hz, theperiod between horizontal sweeps is 65 us. With a period of 1 us forhorizontal retrace, the effective time for each horizontal sweep is 64us. The spacing between points .1 and K is 25 percent of display widthor 64/4 16 us. The pulse duration of multivibrator 77 has been limitedto 0.25 us, so that during the course of 64 frames a correspondingnumber of bright-up pulses may be provided along the line between pointsJ and K without any overlaps. This is shown in FIG. 4a for thehorizontal scan line indicated generally by the reference numeral 128+.With an output from counter 66 of 63, the bright-up pulse provided bymultivibrator 77 during the 128+ horizontal scan line will be initiatedat 62.11 percent of width from the left-hand margin and will beterminated at 62.5 percent of display width from the left-hand margincorresponding to point K.

Each pulse provided by multivibrator 77 is not only applied through ORcircuit 91 to the intensity input of tube but also enables gate 34 tocouple the output of the secondary winding of transformer 22 to low-passfilter 40. If the display is above center, then the beam current will becollected by quadrant and a negative output will appear across thesecondary winding of transformer 22. On the other hand, if the displayis below center, then the beam current will be collected by quadrant 18and the output across the secondary winding of transformer 22 will bepositive. The beam diameter will appreciably exceed the width of thehorizontal etched line separating quadrants l5 and 18. When the displayis precisely centered, equal portions of the beam current will becollected by quadrants 15 and 18; and the output voltage across thesecondary winding of transformer 22 will be zero. The time-constant oflow-pass filter is four times that of the pulses provided bymultivibrator 77. Accordingly, filter 40 stores the average error overfour successive frames. This provides sufficient integration orsmoothing to minimize the effects of noise. The output of filter 40 isapplied through amplifier 48 and resistor 50 to the vertical deflectioninput to adjust the vertical position of the display so that the 128thscan line of the first field of each frame coincides with the etchedhorizontal line separating quadrants l5 and 18.

In operation of the horizontal centering system of FIG. I, assume framecounter 66 has just been indexed to 0 so that it will retain this countfor two successive fields. Comparator 78 will provide an output forthose horizontal scan lines where the six least significant bit outputsof counter 58 are all zeros. This will occur when counter 58 providescounts of0, 64, 128, and 192. AND circuit 79 provides an output in thepresence of a 64 bit output from counter 58 and in the absence of a 32bit output. Accordingly, AND circuit 79 provides an output when counter58 provides counts ranging between 64 and 95 and also between 192 and223". AND circuit 80 provides an output in the presence of a 32 bitoutput from counter 58 and in the absence of a 64 bit output.Accordingly, AND circuit 80 provides an output when counter 58 providescounts ranging between 32 and 63 and also between 160 and 191. The plusoutput of divide-by-two flip-flop 53 becomes positive at the beginningof each horizontal scan line; and the minus output thereof becomespositive at the midpoint of each horizontal scan line. When the count ofcounter 66 is 0, AND circuit 79, comparator 78, and the minus output offlip-flop 53 cause AND circuit 82 to provide an output at the midpointof each of scan lines 64 and 192. During horizontal scan line 64, the128 bit output of counter 58 is zero; and the output of AND circuit 82is coupled through AND circuit 83 and thence through differentiatingcircuit 85 and diode 87 to trigger multivibrator 89. The 1 us pulseoutput of multivibrator 89 is coupled through OR circuit 91 to theintensity input of tube 10 which produces the short horizontal bright-upline B. During horizontal scan line 192, the 128 bit output of counter58 is one; and the output of AND circuit 82 is coupled through AND circuit 84 and thence through differentiating circuit 86 and diode 88 totrigger multivibrator 90. The l as output pulse of multivibrator 90 iscoupled through OR circuit 91 to the intensity input of display tube 10which produces on the screen the short horizontal bright-up line E.

Duringthe second field of the same frame, the count of counter 66 willremain 0. Multivibrator 89 will again provide a bright-up line at themidpoint of the 64th scan line; and multivibrator 90 will again providea bright-up line at the midpoint of the l92nd scan line. However,because of the interlacing signal from divide by-two flip-flop 92 whichis coupled through resistor 69, the scan lines for the second field ofeach frame are displaced below those of the first frame by half thedistance between adjacent scan lines thereof. Accordingly, the bright-uppulse of multivibrator 89 dur ing the second field is slightly belowline B; and the bright-up pulse of multivibrator 90 is slightly belowline E During each field, there are provided two short horizontalbright-up lines which are separated by 50 per cent of raster height.

For both fields of the next frame, the count of counter 66 will be lComparator 78 will provide an output when counter 58 provides counts ofl 65, 129", and 193. At the midpoint of the 65th scan line multivibrator89 will provide a bright-up line which is further below line B; and atthe midpoint of the l93rd scan line, multivibrator 90 will provide abright-up line further below line B.

When the output of frame counter 66 is 31, comparator 78 will provide anoutput when the count of counter 58 is 31", 95, 159, and 223. ANDcircuit 79 causes AND circuit 82 to provide an output at the midpoint ofthe 95th and 223rd scan lines. During the second field of this framewhere the output of counter 66 is 31, multivibrator 89 provides thebright-up line C; and multivibrator 90 provides the bright-up line F.

During the next frame the output of counter 66 is 32. Comparator 78 willprovide an output when counter 58 provides counts of 32, 96, 160 and g224. AND circuit 80 now causes AND circuit 82 to provide an output atthe midpoint of horizontal scan lines 32 and 160. During the first fieldof this frame where the output of counter 66 is 32, multivibrator 89provides the bright-up line A; and multivibrator 90 provides thebright-up line D. During the second field of this frame, multivibrator89 provides a bright-up line slightly below line A; and multivibrator 90provides a bright-up line slightly below line D.

When the output of frame counter 66 is 63, comparator 78 will provide anoutput when the count of counter 58 is 63, 127, 191, and 255. ANDcircuit 80 causes AND circuit 82 to provide an output at the midpoint ofthe 63rd and 191st horizontal scan lines. During the second field ofthis frame where the output of counter 66 is 63", multivibrator 89provides a bright-up line slightly above line B; and multivibrator 90provides a bright-up line slightly above line E.

During the next frame, counter 66 is recycled to the count of During thefirst field of this frame, mul tivibrator 89 again provides thebright-up line B; and multivibrator 90 again provides the bright-up lineE. During the course of 64 frames, short bright-up lines will beproduced for every raster line between those containing bright-up linesA and C and also between those containing bright-up lines D and F.However, none of these bright-up lines will be visible in the displaysince any given line appears only once during 64 frames.

Each bright-up pulse provided by multivibrator 89 is not only coupledthrough OR circuit 91 to the intensity input of display tube 10, butalso enables gate 30 to couple the output of the secondary winding oftransformer 20 to low-pass filter 36. Transformer 20 responds to thosebright-up lines between line A and C. If the display is laterallycentered, then the beam current will be initially collected by quadrant12 for a period of 0.5 1.1.8, producing a negative output across thesecondary winding of transformer 20. Subsequently, the beam current willbe collected by quadrant 15 for an equal period of 0.5 as, producing apositive output across the secondary winding of trans former 20. Whenthe display is laterally centered, the

v durations of collection of beam current by quadrants l2 and 15 areequal; and the output across the secondary winding of transformercomprises a full cycle of a symmetrical square wave having equalnegative and positive durations. Since the average value of asymmetrical square wave is zero, the output from low-pass filter 36 willlikewise be zero when the display is laterally centered. Thetime-constant of filter 36 is eight times that of the pulses provided bymultivibrator 89. Since one horizontal bright-up line is provided in theupper portion of the display for each field, filter 36 stores theaverage error over eight successive fields corresponding to foursuccessive frames. This provides sufficient integration or smoothing toextract the average value of the full cycle square wave output acrossthe secondary winding of transformer 20. If the display is to the leftof center, then the beam current will be collected by quadrant 12 for alonger duration than by quadrant 15. Accordingly the output across thesecondary winding of transformer 20 will comprise a square wave pulsewherein the duration of the negative half cycle is longer than that ofthe positive half cycle. The average value of this asymmetrical squarewave will be negative; and low-pass filter 36 will correspondinglyprovide a negative output. On the other hand, if the display is to theright of center, then the beam current will be collected by quadrant 12for a shorter period than quadrant 15. The output across the secondarywinding of transformer 20 will comprise a square wave having a negativehalf cycle which is of shorter duration than the positive half cycle.The

average value of this asymmetrical square wave is positive; and low-passfilter 36 will correspondingly provide a positive output.

Each bright-up line provided by multivibrator is not only coupledthrough OR circuit 91 to the intensity input of display tube 10, butalso enables gate 31 to couple the output of the secondary winding oftransformer 24 to low-pass filter 38. Transformer 24 responds tohorizontal bright-up lines between D and F in the lower portion of thedisplay. When the display is laterally centered the beam current isinitially collected by quadrant 14 for a period of 0.5 as and issubsequently collected by quadrant 18 for a period of 0.5 pts. Theoutput across the secondary winding of transformer 24 is a symmetricalsquare wave comprising a negative pulse immediately followed by apositive pulse of equal duration. The average output of this symmetricalsquare wave is zero; and low-pass filter 38 provides a correspondingoutput. If the display is to the left of center, the output of filter 38will be negative; and if the display is to the right of center theoutput of filter 38 will be positive.

The output of filter 36 is proportional to the lateral deviation of theupper portion of the display, while the output of filter 38 representsthe lateral deviation of the lower portion of the display. The outputsof filters 36 and 38 are combined in summing amplifier 42 to provide anoutput in accordance with the mean lateral deviation of the display. Theoutput of summing amplifier 42 is applied through resistor 44 to thehorizontal deflection input of the display tube to adjust the lateralposition of the display.

If the mean lateral position of the display is correct but the displayis rotated somewhat clockwise from its proper orientation, the outputfrom filter 36 will be positive while the output from filter 38 will benegative. If the mean lateral position of the display is correct, butthe display is rotated somewhat counterclockwise from its properorientation, the output from filter 36 will be negative while the outputfrom filter 38 will be positive. If the display has the proper angularorientation and is rotated neither clockwise nor counterclockwise, thenthe outputs of filters 36 and 38 will be equal irrespective of thelateral position of the display. Differential amplifier 46 responds toany difference in the outputs of filters 36 and 38 to rotate the displayto its proper angular orientation.

It will be noted that since the periods of multivibrators 89 and 90 are1 ,us, the outputs of filters 36 and 38 will be proportional to lateraldeviations of the upper and lower portions of the display up to i 0.5us. Since the effective time for a horizontal sweep is 64 us, theoutputs of filters 36 and 38 will be proportional to lateral deviationsup to 0.5/64 i 0.78 percent of display width. The range of proportionalresponse of the horizontal centering system may be increased byincreasing the pulse duration of multivibrators 89 and 90 andcorrespondingly increasing the time-constants of filters 36 and 38. Forexample, the range of proportional response of the horizontal centeringsystem may be doubled by increasing the pulse duration of multivibrators89 and 90 to 2 us, and by correspondingly increasing the time-constantsof filters 36 and 38 to 16 us. However, the range of proportionalresponse of the vertical centering system is ordinarily restricted toappreciably smaller values. With 512 horizontal scan lines per frame,the diameter of the beam will be somewhat less than 0.2 percent ofraster height. Accordingly the range of proportional response of thevertical centering system will be somewhat less that $0.1 percent ofraster height. Because of the appreciably larger range of proportionalresponse of the horizontal centering system as compared with thevertical centering system, horizontal centering errors at two verticallyspaced points have been used to correct for rotational errors in thedisplay. The arrangement shown is to be preferred over a configurationwhere vertical errors measured at two spaced horizontal points are usedto correct for rotational errors.

For high accuracy of horizontal centering it is desired that thebright-up lines be placed adjacent the center of the display. However,for'high accuracy in measurement of rotational errors it is desired thatthe horizontal bright-up lines be placed adjacent the upper and lowermargins of the display to maximize the distance between each pair ofbright-up lines. If it is desired to reduce rotational errors at theexpense of slightly increased horizontal centering errors, then theupper bright-up lines could be displaced upwardly to occupy the regionfrom to percent of display height from the upper margin of the display;and the lower bright-up lines could be displaced downwardly to occupythe region from 75 to 100 percent of display height from the uppermargin of the display. If it is desired to reduce horizontal centeringerrors at the expense of slightly increased rotational errors, then theupper bright-up lines could be moved downwardly to occupy the regionfrom 25 to 50 percent of display height from the upper margin of thedisplay; and the lower bright-up lines could be moved upwardly to occupythe region from 50 to 75 percent of display height from the upper marginof the display.

It will be appreciated that horizontal centering errors will beminimized when a group of bright-up lines occupies a central regionextending for example from 37.5 to 62.5 percent of display height fromthe upper margin of the display. Where rotational errors are also to becorrected, then a second and peripheral group of bright-up lines mayextend either from O to 25 percent or from 75 to 100 percent of displayheight from the upper margin of the display. With such arrangement,horizontal centering errors are determined only from the central groupof bright-up lines; and summing amplifier 42 would be replaced by asingle input amplifier. To correct for rotational errors, amplifier 46would respond to the difference in horizontal positions of the centraland peripheral groups of bright-up lines. If desired there may beprovided three groups of brightup lines comprising an upper group, acentral group, and a lower group extending respectively from 0 to 25percent, from 37.5 to 62.5 percent, and from 75 to 100 percent ofdisplay height from the upper margin of the display. Again only thecentral group would be used to control the horizontal position of thedisplay. Rotational errors are corrected by causing amplifier 46 torespond to the difference in horizontal positions for the upper andlower groups of bright-up lines. Such arrangement minimizes bothhorizontal centering errors and rotational errors. 4

The period between successive positive pulses coupled through diode 55is 65 us. The minus output of divide-by-two flip-flop 53 becomespositive 32.5 ts subsequent to each pulse through diode 55. Since theretrace period for horizontal sweep generator 56 is 1 #8, its outputbegins rising from zero volts 1 #8 subsequent to each retrace pulsethrough diode 55. Accordingly, the minus output of divide-by-twoflipflop 53 becomes positive 31.5 as after the output of horizontalsweep generator 56 begins rising from zero volts. Thus the 1 [LS pulsesof multivibrators 89 and 90 extend from 31.5 [1.8 to 32.5 ,us after theoutput of horizontal sweep generator 56 begins rising from zero volts.Since the effective period for a horizontal sweep is 64 us, thehorizontal center of the raster will occur 32 us after the output ofhorizontal sweep generator 56 begins rising from zero volts. Hence theoutput pulses of multivibrators 89 and 90 extend from 0.5 as to the leftof center of the raster to 0.5 as to the right of the center of theraster; and the midpoint of each bright-up line will be horizontallycentered in the raster. From the foregoing, it will be appreciated thatthe midpoint of each bright-up line will be centered in the raster, ifthe retrace period of horizontal sweep generator 56 is made equal to theduration of the pulses provided by multivibrators 89 and 90. Forexample, if the range of proportional response of the horizontalcentering system is doubled by increasing the pulse duration of.multivibrators 89 and 90 to 2 [L8, then the retrace period forhorizontal sweep generator 56 should correspondingly be increased to 2us. Since this will decrease the effective period for a horizontal sweepto 63 ps, the pulse duration of multivibrator 77 may be slightlydecreased to 0.25(63/64) 0.246 [L8 to avoid overlap of the bright-uppulses for the vertical centering system.

From FIG. 4a it will be noted that scan line 127-, which represents thel27th scan line of the second field of each frame, is slightly above thecenter of the raster while the 128+ scan line, which corresponds to the128th scan line of the first field of each frame, is slightly below thecenter of the raster. Since vertical centering is controlled by thebright-up pulses provided on scan line 128+, the raster will not beprecisely centered vertically if the horizontal line separatingquadrants 15 and 18 passes through the center of the display. While theerror is small and perhaps negligible, the raster will be displacedupwardly from center by half the distance between adjacent frame scanlines. With 512 horizontal scan lines per frame, the raster will thus bedisplaced upwardly from center by 0.1 percent of raster height. Ifdesired, this small error may be corrected by displacing the horizontalline separating quadrants 15 and 18 downwardly from the center of thescreen by 0.1 percent of raster height.

Referring now to FIG. 2 there is shown an embodiment of my invention inwhich the metalized screen is etched to provide only three sectors.Sectors l5 and 18 are identical to quadrants 15 and 18 of FIG. 1. Sector13 occupies nearly half of the display area, since the horizontal etchedline separating sectors 15 and 18 is terminated at sector 13. Anodepotential source 26 is connected to one terminal of the primary windingof each of transformers 21, 23, and 25, the other terminals of which areconnected respectively to sectors 13, 15, and 18. One terminal of eachof the secondary windings of transformers 21, 23, and 25 is grounded.The other terminal of the secondary winding of transformer 21 is coupledto the plus input of each of differential amplifiers 95 and 96. Theother terminal of the secondary winding of transformer 23 is connectedto the minus input of each of differential amplifiers 95 and 97. Theother terminal of the secondary winding of transformer 25 is connectedto the minus input of differential amplifier 96 and to the plus input ofdifferential amplifier 97. The outputs of differential amplifiers 95,96, and 97 are coupled through gates 30, 31, and 34 to low-pass filters36, 38, and 40 in the manner shown in FIG. 1.

In operation of the embodiment of FIG. 2, sector 13 encompasses theentire area of sectors 12 and 14 of FIG. 1 and cooperates with sector todetermine horizontal centering errors in the upper portion of thedisplay and cooperates with sector 18 to determine horizontal centeringerrors in the lower portion of the display. Again as in FIG. 1, sectors15 and 18 cooperate to indicate vertical centering errors. In FIG. 1,the provision of four electrically discrete sectors permits the use ofcenter-tapped primary windings to measure the differential currentsbetween adjacent sectors. In FIG. 2, however, the use of center-tappedprimary windings would result in the effective short-circuiting of allsectors since there are an odd number of only three sectors.Accordingly, in FIG. 2, the currents collected by the sectors are sensedby isolated transformers; and the differential currents are measured bydifferential amplifiers 95, 96, and 97.

Referring now to FIG. 3, there is shown an embodiment wherein thediscrete sectors for measuring centering errors occupy a relativelysmall area and wherein there are provided further electrically distinctsectors maintained at a constant potential which collect electron beamcurrent but take no part in the measurement of centering errors. Theembodiment of FIG. 3 provides a higher frequency response, since thestray capacitances of the electrically discrete sectors for measuringcentering errors are considerably reduced. Sectors 15a and 18a abutalong a horizontal line and have a relatively small vertical extent.These sectors are coupled to the terminals of the center-tapped primarywinding of transformer 22. Sectors 12 and 15 abut along a vertical linein an upper region of the display and have a relatively small horizontalextent. These sectors are connected to the terminals of the centertappedprimary winding of transformer 20. Sectors 14 and 18 abut in the lowerregion of the display along an extension of the vertical line ofabutment of sectors 12 and 15. Sectors 14 and 18 have a relatively smallhorizontal extent and are connected to the terminals of thecenter-tapped primary winding of transformer 24. An electricallydistinct sector 17 occupies the area in the upper right-hand portion ofthe display. An electrically distinct sector 19 occupies the area in thelower right-hand portion of the display. An electrically distinct sector13 occupies the area in the left-hand portion of the display. Sectors17, 19, and 13 are connected to the anode source 26; and the areas ofthese sectors are large compared with that of any of the discretemeasuring sectors coupled to the primary windings of the transformers.The embodiment of FIG.

3 operates in the same manner as FIG. 1, but provides an appreciablyhigher frequency response because of the reduction in area of thesectors for measuring positioning errors which results in acorresponding reduction of stray capacitances. The areas of the sectorsfor measuring positioning errors may be made fairly small, since thepositioning errors will usually comprise relatively small percentages ofdisplay width or display height.

Referring now to FIG. 4, there is shown an embodiment of my invention inwhich horizontal centering er rors are minimized by measurement of sucherrors in a central region symmetrically disposed above and below thecenter of the display. Electrically discrete sectors 12, 14, 15, and 18abut along horizontal and vertical lines intersecting adjacent thecenter of the display. Sectors 12 and 14 cooperate to measure verticalcentering errors in a region to the left of the center of the display.Sectors l5 and 18 cooperate to provide vertical centering errors in aregion to the right of the center of the display. Sectors 12 and 15cooperate to indicate horizontal centering errors in a region above thecenter of the display; and sectors 14 and 18 cooperate to providehorizontal centering errors in a region below the center of the display.

Again as in FIG. 3, electrically distinct sectors 13, 17, and 19 areeach connected to the anode source 26. These three sectors aremaintained at a fixed potential and play no part in the measurement ofcentering errors. Sectors 12, 14, 15, and 18 each occupy a relativelysmall portion of thetotal display area to provide low stray capacitanceand hence high frequency response. In FIG. 4, horizontal centeringerrors are measured in only one region; and consequently, rotationalerrors cannot be sensed. If it were desired to sense rotational errors,then as previously indicated horizontal positioning errors could also bemeasured in a region adjacent either the upper or the lower margin ofthe display. Alternatively, horizontal positioning errors might bemeasured adjacent both the upper and lower margins of the display.Source 26 is connected to one terminal of the primary winding of each oftransformers 21 23, 25 and 27. The other terminal of the primary windingof transformer 21 is connected to sector 12; the other terminal of theprimary winding of transformer 23 is connected to sector 15; the otherterminal of the primary winding of transformer 25 is connected to sector18; and the other terminal of the primary winding of transformer 27 isconnected to sector 14. One terminal of the secondary winding of each oftransformers 21, 23, 25, and 27 is grounded. The other terminal of thesecondary winding of transformer 21 is connected to a plus" input ofeach of amplifiers a and 97a. The other terminal of the secondarywinding of transformer 23 is connected to a minus input of amplifier 95aand to a plus input of amplifier 97a. The other terminal of thesecondary winding of transformer 25 is connected to minus inputs ofamplifier 95a and 97a. The other terminal of the secondary winding oftransformer 27 is connected to a plus input of amplifier 95a and a minusinput of amplifier 97a. Amplifiers 95a and 97a 3 filters 36 and 40 asshown in FIG. 1. Since rotational errors are not corrected in theembodiment of FIG. 4, the output of low-pass filter 36 may be applied toa single input amplifier 42a (not shown) of a construction similar toamplifier 48; and the output of amplifier 42a is coupled throughresistor 44 to the horizontal deflection input I-I. Differentialamplifier 46 and summing amplifier 42 of FIG. 1 are not requiredcomponents and may be omitted. In FIG. 4, tube is provided with postdeflection acceleration to achieve high deflection sensitivity and highvisual sensitivity. The conductive aquadag coating on the inner sidewall of the tube is maintained at a potential of 3 Kv by source 26b. Theaquadag conductive coating comprises the accelerating electrode; and themetallized screen, which isat the 10 Kv potential of source 26,comprises the intensifying electrode. As is well known in the art ofpost deflection acceleration tubes, the intensifying electrode may beoperated at a potential of between approximately two and four times thatof the accelerating electrode. From FIGS. 1 and 4 it will be seen thatthe potential of the metallized screen may exceed that of the aquadagcoat ing by an amount ranging from 10 or volts up to many kilovolts.

In FIG. 4 as in FIG. 1, the outputs of AND circuit 73 and differentialamplifier 72 are applied to the inputs of AND circuit 74. The output ofAND circuit'74 is coupled through differentiating circuit 75 andforwardly through diode 76 to multivibrator 77. The output ofmultivibrator 77 actuates gate 34 and is coupled to one input of ORcircuit 91a. The 32 and 64 bit outputs of counter 58 are applied toenabling inputs of AND circuit 79a and to inhibiting inputs of ANDcircuit 80a. The 128 bit output of counter 58 is applied to aninhibiting input of AND circuit 79a and to an enabling input of ANDcircuit 80a. The outputs of AND circuit 79a and 80a are applied to theinputs of OR circuit 81, the output of which is applied to one input ofAND circuit 82. AND circuit 82 receives further inputs from comparator78 and from the minus output of divideby-two flip-flop 53 as shown inFIG. 1. The output of AND circuit 82 is coupled to one input of an ANDcircuit 83a, which receives an inhibiting input from the output of ANDcircuit 73. The output of AND circuit 83a is coupled throughdifferentiating circuit 85 and forwardly through diode 87 tomultivibrator 89. The output of multivibrator 89 enables gate 30 and iscoupled to a second input of OR circuit 91a, the output of which isapplied to the intensity input of tube 10.

In operation of the vertical centering system of FIG. 4, multivibrator77 provides a bright-up pulse once each frame which is coupled throughOR circuit 91a to the intensity input of tube 10. This bright-up pulseis provided during the 128+ horizontal scan line. The bright-up pulseprovided by multivibrator 77 also enables gate 34 to respond to theoutput of amplifier 970. When the output of frame counter 66 is 0 thebrightup pulse is provided at position J. When the output of framecounter 66 is 63 the bright-up pulse will be provided at position K. Foroutputs of frame counter 66 ranging between 0 and 31 the bright-uppulses of multivibrator 77 will be to the left of the center of thedisplay; and vertical centering errors are measured by the difference incurrents drawn by sectors 12 and 14. The secondary windings oftransformers 21 and 27 are coupled to respective plus and minus inputsof amplifier 970. When the display is vertically centered, equalcurrents are drawn by sectors 12 and 14; and equal voltages appearacross the secondary windings of transformers 21 and 27. Thus, when thedisplay is centered vertically, amplifier 97a provides no output. If thedisplay is above center, then more beam current will be collected bysector 12 than by sector 14. The negative output across the secondarywinding of transformer 21 will exceed the negative output across thesecondary winding of transformer 27; and the output of amplifier 97awill be negative. On the other hand if the display is below center thenmore beam current will be collected by sector 14 than by sector 12. Thenegative output across the secondary winding of transformer 27 willexceed the negative output across the secondary winding of transformer21; and the output of amplifier 97a will be positive.

For outputs of frame counter 66 ranging between 32 and 63 the bright-uppulses of multivibrator 77 will be-provided to the right of the centerof the display.

Vertical centering errors are now measured by the difference in currentsdrawn by sectors 15 and 18. It will be noted that the secondary windingsof transformers 23 and 25'are connected to respective plus" and minusinputs of amplifier 97a. If the display is vertically centered equalcurrents are drawn by sectors 15 and 18; and equal negative voltagesappear across the secondary windings of transformers 23 and 25. Thus theoutput of amplifier 97a will be zero. If the display is above center,then the output of amplifier 97a will be negative; and if the display isbelow center than the output of amplifier 97a will be positive. In theforegoing description it has been assumed that the display ishorizontally centered. However, the vertical centering system stilloperates properly even if the display is not horizontally centered. Ifthe display is slightly to the left of center, then sectors 12 and 14will measure vertical centering errors for outputs of frame counter 66ranging between 0 and 32 while sectors 15 and 18 will measure verticalcentering errors for outputs of frame counter 66 ranging between 33 and63. If the display is slightly to the right of center, then sectors 12and 14 will measure vertical centering errors for outputs of framecounter 66 ranging between 0 and 30, while sectors 15 and 18 willmeasure vertical centering errors for outputs of frame counter 66ranging between 31 and 63.

In operation of the horizontal centering system of FIG. 4, assume framecounter 66 has just been indexed to 0. As in FIG. 1, comparator 78 willprovide an output when counter 58 provides counts of 0, 64, 128, and192. AND circuit 79a provides an output when the output of counter 58ranges between 96 and 127; and AND circuit 800 provides an output whenthe output of counter 58 ranges between 128 and 159. Again the minusoutput of divide-by-two flip-flop 53 becomes positive at the mid-pointof each horizontal scan line. During the first field of the frame wherethe output of frame counter 66 is 0, AND circuit a, comparator 78, andthe minus output of flip-flop 53 cause AND circuit 82 to provide anoutput at the mid-point of scan line 128+. However, during the 128+horizontal scan line, AND circuit 73 provides an output which inhibitsAND circuit 83a from coupling the output of AND circuit 82 tomultivibrator 89. Thus,

no horizontal bright-up line may be provided during the 128+ scan line.As may be seen by reference to FIG. 4a, scan line 128+ is reserved forthe bright-up pulses of the vertical centering system.

During the second field of this frame where the output of frame counter66 is AND circuit 82 will provide an output at the mid-point of scanline 128. Since AND circuit 73 does not provide an output, AND circuit83a is enabled; and the output of AND circuit 82 is coupled through ANDcircuit 83a, differentiating circuit 85, and diode 87 to triggermultivibrator 89. The 1 us pulse output of multivibrator 89 is coupledthrough OR circuit 91a to the intensity input of tube which produces thehorizontal bright-up line 128- as shown in FIG. 4a.

For both fields of the next frame, the output of frame counter 66 willbe 1". Comparator 78 willprovide an output when counter 58 providesoutputs of l 65 I29, and 193. For the first field of this frame, ANDcircuit 82 will trigger multivibrator 89 to provide the bright-up line129+ of FIG. 4a; and during the second field of this frame, AND circuit82 will trigger multivibrator 89 to provide the bright-up line 129 ofFIG. 4a.

When the output of frame counter 66 is 31, comparator 78 will provide anoutput when the count of counter 58 is 31, 95, I59, and 223. During thefirst field of this frame, AND circuit 80a causes multivibrator 89 toprovide a bright-up pulse at the mid-point of the 159+ scan line whichlies just above line C of FIG. 4. During the second field of this frame,multivibrator 89 provides the bright-up line C durin the 159 scan line.

During the next frame the output of counter 66 is 32. Comparator 78 willprovide an output when counter 58 provides counts of 32, 96, 160, and224. AND circuit 79a now causes AND circuit 82 to provide an output atthe mid-point of horizontal scan line 96. During the first field of thisframe, multivibrator provides the bright-up line A during the 96+ scanline. During the second field of this frame, multivibrator 89 provides abright-up line slightly below line A.

When the output of frame counter 66 is 63, comparator 78 will provide anoutput when the count of counter 58 is 63, l27, 191, and 255. ANDcircuit 79a causes AND circuit 82 to provide an output at the mid-pointof horizontal scan line 127. During the first field of this framemultivibrator 89 provides the bright-up line 127+ shown in FIG. 4a; andduring the second field of this frame multivibrator 89 provides thehorizontal bright-up line 127- as shown in FIG. 4a.

During the next frame counter 66 is recycled to the count of 0. Duringthe first field of this frame, AND circuit 73 inhibits AND circuit 83aso that multivibrator 89 provides no horizontal bright-up line. Thefirst field of this frame is reserved for the short bright-up pulses ofthe vertical centering system which appear on horizontal scan line 128+.During the second field of this frame multivibrator 89 provides thehorizontal bright-up line 128- as shown in FIG. 4a. During the course of64 frames, short horizontal bright-up lines will be produced for everyraster line between those containing bright-up lines A and C except scanline 128+.

Each bright-up pulse provided by multivibrator 89 is not only coupledthrough OR circuit 91a to the intensity input of display tube 10 butalso enables gate 30 to couple the output of amplifier 95a to low-passfilter 36. For those bright-up lines above the center of the display,the beam current will be initially collected by quadrant 12, producing anegative output across the secondary winding of transformer 21. Sincethis output is coupled to a plus" input of amplifier 950, a negativeoutput will be initially produced therefrom. Subsequently the beamcurrent will be collected by quadrant l5, producing a negative outputacross the secondary winding of transformer 23. Since this output iscoupled to a minus input of amplifier 950, the output therefrom will bepositive. If the display is laterally centered the duration ofcollection of beam current by quadrants 12 and 15 will be equal; and theoutput of amplifier 95a will comprise a full cycle of a symmetricalsquare wave having equal negative and positive durations. Accordinglythe average value of the output of amplifier 95a will be zero. If thedisplay is to the left of center, then the average value of the outputof amplifier 95a will be negative, while if the display is to the rightof center then the average value of the output of amplifier 95a will bepositive.

For those bright-up lines below the center of the display, the beamcurrent will be initially collected by quadrant 14 producing a negativeoutput across the secondary winding of transformer 27. Since this outputis coupled to a plus input of amplifier 95a the output thereof willinitially be negative. Subsequently the beam current will be collectedby quadrant 18, producing a negative output across the secondary windingof transformer 25. Since this output is coupled to a minus input ofamplifier 95a, the output thereof will become positive. When the displayis laterally centered, the durations of collection of beam currents byquadrants 14 and 18 are equal; and the output of amplifier 95a comprisesa full cycle of a symmetrical square wave having equal negative andpositive durations. The average value of this symmetrical square wave iszero. If the display is to the left of center then the average value ofthe output of amplifier 95a will be negative, while if the display is tothe right of center then the average value of the output of amplifier95a will be positive. Filter 36 extracts the average value of the outputof amplifier 95a and applies it to the single input amplifier 42a(previously described) and thence through register 44 to the horizontaldeflection input of display tube 10.

Since in FIG. 4 rotational errors are not sensed, it is not necessary toprovide a wide range of proportional response for the horizontalcentering system. Accordingly if desired, the range of proportionalresponse may be reduced by reducing the pulse duration of multivibrator89 and correspondingly reducing the timeconstant of filter 36. Forexample the range of proportional response of the horizontal centeringsystem may be halved by reducing the pulse duration of multivibrator 89to 0.5 us and by correspondingly reducing the time-constant of filter 36to 4 ,lLS. In such event when the display is laterally centered, sectors12 and 15 and sectors 14 and 18 will each collect beam current for aperiod of 0.25 us, which is one-half the pulse period of multivibrator89. Since the horizontal and vertical centering systems are both subjectto the same frequency restrictions, brighttup lines for the horizontalcentering system should be not less than twice as long as the bright-uppluses for the vertical centering system. Accordingly the pulse durationof multivibrator 89 should be at least twice that of multivibrator 77.

It will be seen that the objects of my invention have been accomplished.My display centering system employs a metallized screen split alonglines passing adjacent the center of the display. Bright-up pulses forthe vertical centering system and bright-up lines for both thehorizontal centering system and the rotational correction system areprovided at positions in the raster which vary from field to field andfrom frame to frame so as not to be visible in the display. Highlinearity of the deflection system is not required to maintain thecenter of the display adjacent the'center of the tube; and the entiredisplay area is available for presenting information.

It will be understood that certain features and subcombinations are ofutility and may be employed without reference to other features andsubcombinations. It is further obvious that various changes may be madein details without departing from the spirit of my invention. It istherefore to be understood that my invention is not to be limited to thespecific details shown and described.

Having thus described my invention, what I claim is:

1. In a display centering system, the combinationincluding a cathode raytube having horizontal and vertical deflection inputs and an intensityinput, means for applying sweep signals to said deflection inputs toprovide a raster having a certain frame repetition frequency, and firstmeans for pulsing the intensity input at a predetermined point in theraster during each frame and for successively shifting the position ofsaid point from frame to frame.

2. A system as in claim 1 wherein the first means is so constructed thatsaid points lie along a horizontal line adjacent the center of theraster.

3. A system as in claim 1 wherein the first means is so constructed thatsaid points lie along a vertical line adjacent the center of the raster.

4. A system as in claim 1 wherein the first means is so constructed thatsaid points lie remote the center of the raster along a vertical lineextending adjacent the center thereof.

5. A system as in claim 1 wherein the first means is so constructed thatsaid points lie along a horizontal line and wherein the first meansprovides pulses of relatively short duration.

6. A system as in claim 1 wherein the first means is so constructed thatsaid points lie along a vertical line and wherein the first meansprovides pulses of relatively long duration.

7. A system as in claim 1 further including second means for pulsing theintensity input at a certain point in the raster during each frame andfor successively shifting the position of said certain point from frameto frame.

8. A system as in claim 7 wherein the first means is so constructed thatsaid predetermined points lie along a horizontal line and wherein thesecond means is so constructed that said certain points lie along avertical line.

9. A system as in claim 8 wherein the second means provides pulses ofappreciably longer duration than those provided by the first means.

10. A system as in claim 7 wherein the first means is so constructedthat said predetermined points lie in a first region along a verticalline and wherein the second means is so constructed that said certainpoints lie in a second and different region along said vertical line.

11. A system as in claim 10 wherein the first and second means providepulses of substantially equal durations.

12. A system as in claim 10 wherein the first region is entirely abovethe center of the raster and the second region is entirely below thecenter of the raster.

13. A system as in claim 10 wherein the first region has substantiallyequal extents above and below the center of the raster and the secondregion is adjacent one of the upper and lower margins of the raster.

14. A system as in claim 10 wherein the regions are of substantiallyequal vertical extents.

15. A system as in claim 10 wherein the regions have vertical extentswhich do not exceed one-third of raster height.

16. A display centering system including in combination a cathode raytube having horizontal and vertical beam deflection inputs and a displayrotation input and a metallized screen divided into at least twoelectrically discrete sectors, means for collecting beam current fromeach sector, means responsive to the currents collected from the sectorsfor providing a control signal, and means coupling the control signal tothe rotation input.

17. A display centering system including in combination a cathode raytube having horizontal and vertical beam deflection inputs and ametallized screen divided into at least two electrically discretesectors, means for collecting beam current from each sector, meansincluding a gate and responsive to the currents collected from thesectors for providing a control signal, and means for momentarilyenabling the gate at predetermined intervals of time.

18. A display centering system including in combination a cathode raytube having horizontal and vertical beam deflection inputs and ametallized screen divided into at least two electrically discretesectors, means for collecting beam current from each sector, meansresponsive to the currents collected from the sectors for providing acontrol signal, an algebraic combining device having a plurality ofinputs and means including a gate and a low-pass filter coupling thecontrol signal to one input of said plurality.

19. A display centering system including in combination a cathode raytube having horizontal and vertical beam deflection inputs and ametallized screen divided into at least two electrically discretesectors, means for collecting beam current from each sector, meansresponsive to the currents collected from the sectors for providing acontrol signal, an algebraic combining device having a plurality ofinputs, means coupling the control signal to one input of saidplurality, a low-pass filter and means comprising a gate for couplingthe device to the filter.

20. A system as in claim 16 including means coupling the control signalto the horizontal deflection input.

21. A system as in claim 17 wherein the tube is provided with anintensity input, the system further including means responsive to theenabling means for exciting the intensity input.

22. A system as in claim 17 including a low-pass filter and meanscoupling the gate to the filter.

23. In a display centering system, the combination including a cathoderay tube having horizontal and vertical deflection inputs and anintensity input, means for applying horizontal and vertical sweepsignals to the respective deflection inputs to provide a raster having acertain frame repetition frequency, means including a recycling countingdevice responsive to, the sweep signal means for providing a digitalindication which changes from frame to frame, and means responsive tothe counting device for controlling the intensity input.

24. A system as in claim 23 wherein the control means includes adigital-to-analog converter responsive 1

1. In a display centering system, the combination including a cathoderay tube having horizontal and vertical deflection inputs and anintensity input, means for applying sweep signals to said deflectioninputs to provide a raster having a certain frame repetition frequency,and first means for pulsing the intensity input at a predetermined pointin the raster during each frame and for successively shifting theposition of said point from frame to frame.
 2. A system as in claim 1wherein the first means is so constructed that said points lie along ahorizontal line adjacent the center of the raster.
 3. A system as inclaim 1 wherein the first means is so constructed that said points liealong a vertical line adjacent the center of the raster.
 4. A system asin claim 1 wherein the first means is so constructed that said pointslie remote the center of the raster along a vertical line extendingadjacent the center thereof.
 5. A system as in claim 1 wherein the firstmeans is so constructed that said points lie along a horizontal line andwherein the first means provides pulses of relatively short duration. 6.A system as in claim 1 wherein the first means is so constructed thatsaid points lie along a vertical line and wherein the first meansprovides pulses of relatively long duration.
 7. A system as in claim 1further including second means for pulsing the intensity input at acertain point in the raster during each frame and for successivelyshifting the position of said certain point from frame to frame.
 8. Asystem as in claim 7 wherein the first means is so constructed that saidpredetermined points lie along a horizontal line and wherein the secondmeans is so constructed that said certain points lie along a verticalline.
 9. A system as in claim 8 wherein the second means provides pulsesof appreciably longer duration than those provided by the first means.10. A system as in claim 7 wherein the first means is so constructedthat said predetermined points lie in a first region along a verticalline and wherein the second means is so constructed that said certainpoints lie in a second and different region along said vertical line.11. A system as in claim 10 wherein the first and second means providepulses of substantially equal durations.
 12. A system as in claim 10wherein the first region is entirely above the center of the raster andthe second region is entirely below the center of the raster.
 13. Asystem as in claim 10 wherein the first region has substantially equalextents above and below the center of the raster and the second regionis adjacent one of the upper and lower margins of the raster.
 14. Asystem as in claim 10 wherein the regions are of substantially equalvertical extents.
 15. A system as in claim 10 wherein the regions havevertical extents which do not exceed one-third of raster height.
 16. Adisplay centering system including in combination a cathode ray tubehaving horizontal and vertical beam deflection inputs and a displayrotation input and a metallized screen divided into at least twoelectrically discrete sectors, means for collecting beam current fromeach sector, means responsive to the currents collected from the sectorsfor providing a control signal, and means coupling the control signal tothe rotation input.
 17. A display centering system including incombination a cathode ray tube having horizontal and vertical beamdeflection inputs and a metallized screen divided into at least twoelectrically discrete sectors, means for collecting beam current fromeach sector, means including a gate and responsive to the currentscollected from the sectors for providing a control signal, and means formomentarily enabling the gate at predetermined intervals of time.
 18. Adisplay centering system including in combination a cathode ray tubehaving horizontal and vertical beam deflection inputs and a metallizedscreen divided into at least two electrically discrete sectors, meansfor collecting beam current from each sector, means responsive to thecurrents collected from the sectors for providing a control signal, analgebraic combining device having a plurality of inputs and meansincluding a gate and a low-pass filter coupling the control signal toone input of said plurality.
 19. A display centering system including incombination a cathode ray tube having horizontal and vertical beamdeflection inputs and a metallized screen divided into at least twoelectrically discrete sectors, means for collecting beam current fromeach sector, means responsive to the currents collected from the sectorsfor providing a control signal, an algebraic combining device having aplurality of inputs, means coupling the control signal to one input ofsaid plurality, a low-pass filter and means comprising a gate forcoupling the device to the filter.
 20. A system as in claim 16 includingmeans coupling the control signal to the horizontal deflection input.21. A system as in claim 17 wherein the tube is provided with anintensity input, the system further including means responsive to theenabling means for exciting the intensity input.
 22. A system as inclaim 17 including a low-pass filter and means coupling the gate to thefilter.
 23. In a display centering system, the combination including acathode ray tube having horizontal and vertical deflection inputs and anintensity input, means for applying horizontal and vertical sweepsignals to the respective deflection inputs to provide a raster having acertain frame repetition frequency, means including a recycling countingdevice responsive to the sweep signal means for providing a digitalindication which changes from frame to frame, and means responsive tothe counting device for controlling the intensity input.
 24. A system asin claim 23 wherein the control means includes a digital-to-analogconverter responsive to the counting device.
 25. A system as in claim 24wherein the control means includes an analog comparator responsive tothe converter and to the horizontal sweep signal.
 26. A system as inclaim 23 wherein the sweep signal means includes a recycling counterproviding a digital indication which changes from line to line of eachframe.
 27. A system as in claim 26 wherein the control means includes adigital comparator responsive to the counting device and to the counter.